Not if its wound field, all you need to do is vary the relatively low field current.What you generally do NOT want to do is throttle the motor power through a pot. The pot would have to be rated to at least the power of the motor and that makes for an expensive pot. It can be done for small-ish motors (like a toy train set) but becomes impractical at larger scales.
I had another answer given to me which was adding the variable resistor in the field winding of the dc shunt motor and you can change the speed by varying the flux. Is this correct?Vary the shunt voltage current is one simple way.
That is if wound field, not PM
right, thanks appreciate it.flux control method.
Lower current = higher RPM